FIG. 13 is a sectional view schematically showing the structure of a conventional general moving-magnet linear motor. A movable element 1310 comprises field permanent magnets 1311 and movable element yoke 1312. The movable element yoke 1312 is arranged on the back surfaces of the permanent magnets 1311. The movable element yoke 1312 has a function of transmitting a magnetic flux generated by the field permanent magnet 1311, and a function of supporting the entire movable element 1310. The permanent magnets 1311 are arranged side by side in the moving direction (right-to-left direction on the sheet surface of FIG. 13) of the movable element 1310. A stator 1320 comprises a stator yoke 1321 formed from a laminated iron core with a comb-like tooth structure, and a coil 1323 which is wound around every other iron-core tooth 1322 of the stator yoke 1321 for three-phase driving. Each iron-core tooth 1322 has an I shape, and the I-shaped iron-core teeth 1322 are also arranged in the moving direction of the movable element 1310. The longitudinal direction of each iron-core tooth 1322 is a direction perpendicular to the sheet surface of FIG. 13. A magnetic field is generated by the coil 1323 which is properly energized via wiring (not shown). The interaction between the magnetic field and a magnetic field generated by the permanent magnet 1311 generates a moving force. In FIG. 13, the laminated iron core which constitutes the movable element yoke 1312 is fixed onto a stator base 1324.
A conventional cooling method applied to the above linear motor will be explained with reference to FIGS. 14 to 16. The temperature of the coil 1323 rises due to heat generated by the coil 1323. When the temperature exceeds the heat resistant temperature of the insulating covering of the coil 1323, the insulating covering is torn. As a result, the coil 1323 short-circuits and is damaged. The first purpose of cooling is to prevent the damage to the coil.
The second purpose of cooling is to control the temperature environment of the linear motor applied to a semiconductor manufacturing apparatus, machine tool, and the like which are required to attain a high precision. Even with such a temperature rise as not to short-circuit the coil, heat by the temperature rise of the coil increases the temperature of the whole linear motor. The temperature rise of the linear motor leads to the temperature rise of a structure, moving table, or the like which supports the linear motor, deforming the structure, moving table, or the like. Deformation of various members must be prevented by controlling the temperature environment by cooling.
As disclosed in International Patent Laid-Open No. 10-511837, in the arrangement shown in FIG. 14, the coil 1323 and a cooling pipe 1401 are arranged at a gap (slot) defined by the I-shaped iron-core teeth 1322 of the stator 1320. The cooling pipe 1401 is made of copper or aluminum. Heat generated by the coil 1323 is transferred to the cooling pipe 1401 and absorbed by a coolant 1402 flowing through the cooling pipe 1401. In order to enhance the heat transfer between the cooling pipe 1401 and the coil 1323 and the endothermic effect, the cooling pipe 1401 and coil 1323 are thermally coupled to each other by a sheet (not shown).
Since the cooling pipe 1401 is a conductor, an eddy current is generated by a leakage flux at the slot which is a gap defined by the iron-core teeth 1322. This eddy current degrades linear motor characteristics. To prevent this, the cooling pipe is arranged at the bottom of the slot (i.e., on the stator base).
As disclosed in Japanese Patent Laid-Open No. 10-257750, in the second prior art shown in FIG. 15, the cooling pipe 1401 is arranged outside the stator yoke (laminated iron core) 1321 and coil 1323. In order to prevent degradation of the performance caused by the eddy current, the cooling pipe 1401 is made of a nonmagnetic material. Part of heat generated by the coil 1323 is directly transferred to the cooling pipe 1401 and absorbed by it. The remaining heat is transferred to the stator yoke (laminated iron core) 1321, then transferred to the cooling pipe 1401, and absorbed by the coolant 1402.
In the third prior art shown in FIG. 16, the cooling pipe 1401 is arranged in the stator base 1324. Since neither leakage magnetic flux nor eddy current is generated at this place, a cooling pipe 1401 of a conductor can be adopted. Heat generated by the coil 1323 reaches the stator base 1324 via the stator yoke (laminated iron core) 1321 in contact with the coil 1323, and absorbed by the coolant 1402 in the cooling pipe 1401. Also in this prior art, a sheet or bobbin (not shown) is interposed between the stator yoke 1321 and the coil 1323 so as to efficiently transfer heat to the stator yoke (laminated iron core) 1321.
In the above-mentioned prior arts, the circumferential portion of the coil 1323 is cooled, and no attention is paid to the temperature distribution (e.g., internal temperature) of the coil 1323. The application limit of the linear motor is determined by the temperature of a portion of the coil where the temperature becomes highest. Even if the temperature near the cooling pipe does not reach the application limit temperature of the winding, but the temperature of a portion of the coil where the temperature becomes highest reaches the application limit temperature, the linear motor cannot be used.
The method shown in FIG. 14 is one-side cooling of cooling only the lower portion of the coil 1323. The upper portion (opposite side) is slightly cooled by air, but the temperature of the interior (particular a portion apart from the cooling pipe) becomes higher than the outer temperature. Note that the cooling pipe cannot be arranged at the upper portion owing to the above-mentioned problem of generating an eddy current.
The method shown in FIG. 15 is also one-side cooling though the nonmagnetic pipe is arranged above the stator yoke (laminated iron core) 1321 and coil 1323. The lower portion (opposite side) of the coil 1323 is the slot bottom, and heat generated by the coil 1323 is transferred to the stator yoke (laminated iron core) 1321 and absorbed. However, the internal coil temperature also rises. In this arrangement, the cooling pipe 1401 is interposed between the stator yoke (laminated iron core) 1321 and the permanent magnet of the movable element. The permanent magnet must be spaced apart from the stator yoke (laminated iron core) 1321 by the thickness of the cooling pipe 1401. The magnetic flux density does not become higher, compared to FIG. 14, and the thrust generated upon supplying the same amount of current to the coil 1323 is smaller. If the current is increased to obtain the same thrust, the heat generation amount of the coil 1323 increases by the square of the current, resulting in a very low efficiency. The permanent magnet may be made thick in order to increase the magnetic flux density. However, the movable element becomes heavy and requires a larger thrust for movement. Even if the thrust constant is increased by simply making the permanent magnet thick, the movable element weight increases much more, increasing heat generated by the coil 1323. Thus, the structure of FIG. 15 is not optimal.
In terms of coil cooling, the arrangement shown in FIG. 16 does not directly cool the coil 1323, and has the lowest cooling efficiency of the three prior arts. Heat which cannot be absorbed by the coolant flowing through the stator is dissipated to air around the linear motor. Part of heat is transferred to a surface plate which supports the stator, and deforms the surface plate. This method can cool the coil in air even with a low efficiency. However, heat cannot be dissipated by air in a linear motor used for a recently demanded step & scan projection exposure apparatus which operates in a vacuum environment and uses EUV (Extreme Ultra Violet) light as exposure illumination light, because this linear motor is set in a vacuum environment. All heat must be absorbed via the cooling pipe, but there is a gap between the laminated steel plate of the stator yoke 1321 and the coil 1323. In the vacuum environment, the gap is vacuum and set in a heat-insulated state. Further, the stator yoke (laminated iron core) 1321 and stator base 1324 macroscopically seem to be in contact with each other, but microscopically have a gap between them. In the vacuum environment, the gap is vacuum and set in a heat-insulated state. In this environment, heat of the coil 1323 is hardly transferred to the coolant 1402. That is, the coil 1323 is placed in almost the heat-insulated state in the vacuum environment. It is substantially difficult to supply a current to the coil 1323, and the linear motor does not function. In the vacuum environment, the arrangements shown in FIGS. 14 and 15 also suffer the same problem.
One factor impeding the stage performance of each of the mask stage and the wafer stage is deformation of the structure caused by heat. Even when a wafer support member and a mask support member constituting parts of the respective stages are each formed of a material exhibiting low thermal expansion, such as SiC, the stage performance is potentially affected unless temperature control is performed at an accuracy level of not larger than 0.001° C. Also, when the wafer and the mask are moved at a high acceleration for the purpose of a higher throughput, heat generated by an electromagnetic motor, e.g., a linear motor for driving the stage, gives rise to a problem. If the stage acceleration is doubled, the generated heat is increased four times because it is in proportion to the square of acceleration. The electromagnetic motor for moving the wafer or the mask over a large stroke is responsible for 90% or more of the heat generated in each stage.
As compared with an apparatus for projecting and exposing a mask pattern to a wafer in air or an inert gas, e.g., nitrogen, the EUV exposure apparatus is advantageous in that, because of projecting and exposing a mask pattern to a wafer in a vacuum, the heat generated from coils of the electromagnetic motor is not transmitted to the mask stage or the wafer stage through the air or the inert gas. In the EUV exposure apparatus, if the heat generated from coils of the electromagnetic motor is avoided from being transmitted to apparatus components, such as a surface plate, through members supporting the mask stage or the wafer stage, there is no necessity of preventing the heat generation from the coils of the electromagnetic motor in order to eliminate an adverse effect upon the stage performance.
However, the above-mentioned advantage of the heat generated from the coils of the electromagnetic motor not being transmitted to the mask stage or to the wafer stage because of the employment of a vacuum means, on the other hand, that the heat generated from the coils of the electromagnetic motor will accumulate in the electromagnetic motor itself. Accordingly, there occurs a problem that the coils of the electromagnetic motor may suffer overheat damage from the heat generated by the coils themselves.